Electronic devices such as laptop PCs, tablet PCs, digital cameras, and mobile phones, which are portable while being used by the user are rapidly increasing in recent years. Many of these electronic devices have therein a rechargeable battery, which requires periodical charging. To facilitate the work for charging the rechargeable battery of an electronic device, there are an increasing number of devices for charging rechargeable batteries by using a power-supplying technology (wireless power transmission technology performing power transmission by varying the magnetic field) that performs wireless power transmission between a power-supplying device and a power-receiving device mounted in an electronic device.
Examples of such a wireless power transmission technology includes: a technology that performs power transmission by means of electromagnetic induction between coils (e.g. see PTL 1) and a technology that performs power transmission by means of resonance phenomena between resonators (coils) provided to the power-supplying device and the power-receiving device (e.g. see PTL 2).
These wireless power transmission technologies, during wireless power transmission, cause a considerable transmission loss as compared with wired power transmission. Reduction of this transmission loss in order to improve the power transmission efficiency (ratio of the power received by the power-receiving device for the power transmitted from the power-supplying device) has been a major issue.
To address this issue, for example, PTL 2 discloses a wireless power transmission apparatus, which even when the distance between the power-supplying resonance coil and the power-receiving resonance coil changes, maintains the resonant state to achieve a high power transmission efficiencies from the power-supplying device to the power-receiving device, by changing the resonance frequencies of power-supplying resonance coil and the power-receiving resonance coil to successively change the coupling strength between the power-supplying resonance coil and the power-receiving resonance coil. Further, PTL 3 discloses a wireless power device capable of improving the power transmission efficiency of the entire device by changing the coupling strength between a power-supplying coil and a power-receiving coil. Further, PTL 4 discloses a power supply system provided with a power-supplying resonance coil and a power-receiving resonance coil between a power-supplying coil and a power-receiving coil, which system detects the distance c between the power-supplying resonance coil and the power-receiving resonance coil when performing contactless power supply and adjusts the distance a between the power-supplying coil and the power-supplying resonance coil and the distance b between the power-receiving coil and the power-receiving resonance coil so as to maximize the power-supplying efficiency according to the distance c detected.